1. Field of the Invention
The present invention relates to thermoplastic elastomer compositions prepared by dynamic vulcanization techniques and, more particularly, relates to thermoplastic olefin (TPO) compositions comprising a crystalline polyolefin resin blended, under conditions of dynamic vulcanization, with a halobutyl rubber material in the presence of a maleimide curative. These thermoplastic elastomer compositions manifest superior heat aging characteristics and can be stabilized to U.V. exposure without adversely affecting other characteristics thereof.
2. Background of the Invention
Polymer blends which have a combination of both elastic and thermoplastic properties are generally obtained by blending an elastomeric composition with a thermoplastic composition in a manner such that the elastomer is intimately and substantially uniformly dispersed as a discrete phase within a thermoplastic continuous phase. Dynamic vulcanization techniques for producing such polymer blend compositions having both elastic and thermoplastic properties are described by Gessler and Haslett in U.S. Pat. NO. 3,037,954. This patent includes disclosure of a process wherein a vulcanizable elastomer is dispersed into a resinous thermoplastic polymer and subsequently cured while continuously mixing and shearing the polymer blend. The result is a micro-gel dispersion of cured rubber in an uncured matrix of resinous thermoplastic polymers.
Thermoplastic olefin (TPO) compositions are polymer blends wherein the elastometric composition includes an olefinic elastomer. These TPO compositions exhibit properties of a cured elastomer as well as the reprocessibility of a thermoplastic resin. The elastomeric characteristics are enhanced if the olefinic elastomer is wholly or partially cross-linked.
Thermoplastic olefin compositions which include a crystalline polyolefin resin blended with a butyl rubber material in the presence of a curing agent and under dynamic vulcanization conditions are known. For example, U.S. Pat. No. 4,130,534 discloses TPO compositions wherein a butyl rubber, preferably halogen-free butyl rubber, is blended with a crystalline thermoplastic polyolefin resin under conditions of dynamic vulcanization and in the presence of a curing agent selected from any curative system applicable for vulcanizing butyl or halobutyl rubber. Disclosures of such curatives include sulfur, phenolic resin, metal oxide, p-quinone dioxime or bismaleimide vulcanizing systems, the phenol-aldehyde resins are said to be preferred.
Although such TPO compositions are capable of withstanding temperatures of up to 150.degree. C. for a relatively short period of time, after four to six weeks elongation significantly declines. Thus, such compositions are not optimal in applications requiring long-term heat aging characteristics such as in automotive under-hood applications, in appliances and in other high temperature environments.
It has now been discovered that utilization of a specific curing system, namely, a maleimide curing system, in combination with a halobutyl elastomer produces TPO compositions having unexpected long-term heat aging characteristics. Thus, the present invention is directed to TPO compositions having improved long-term heat aging characteristics and resides in providing TPO compositions wherein a crystalline polyolefin resin and a halobutyl elastomer are blended together under conditions of dynamic vulcanization and in the presence of a maleimide curative.
It has also been discovered that utilization of such maleimide curing systems in preparing TPO compositions which include a halobutyl rubber and a U.V. stabilizer is particularly suitable for producing ultraviolet stable TPO compositions having long-term heat aging characteristics. It has been found in the investigations leading to this invention that the crosslinking performance of curatives for halobutyl materials is significantly decreased in the presence of U.V. stabilizers. However, it has been surprisingly discovered that particularly for bromobutyl rubber, this decrease occurs to a much lesser degree in the presence of a maleimide curative system.
Additionally, it is an objective within the field of this invention to achieve TPO compositions that are white in color so as to be more attractive in applications such as appliances. Prior art products typically appear only in light beige/cream colors and are not as desirable for such uses.
Maleimide curatives for curing rubber components, other than butyl rubbers, of a blend of rubber and thermoplastic resin are known. See, for example, U.S. Pat. Nos. 3,641,215 and 4,104,210.